The present invention relates generally to apparatus which permit video signals of different scan formats to be superimposed on a common display according to a predetermined priority, and more particularly to a scan line synchronizer for establishing synchronism between horizontal and vertical synchronization pulses of a first video signal and those of a second video signal, there being a difference of (2n-1) horizontal scan lines between the first and second video signals.
Recent advances in IC and LSI technologies have brought about significant cost reduction and improvements in computers. Personal computers, now available at modest prices, find extensive use in businesses and households. With the ever increasing trend toward the widespread use of personal computers, demands have arisen for a device that permits the personal computers to be coupled with an external video source such as television or video recorders for the purpose of superimposing the image of the external source with the computer-generated graphics and characters on a common display unit.
However, the scan formats of the signals generated by computer and external source often differ from one another. A conventional circuit that permits coupling of such signals is costly and only available for special business applications.